a) Field of the Invention
This invention relates to an engine speed control system for a construction machine such as a hydraulic excavator, which is suitable for arrangement in the construction machine and, when directional control valves for controlling drive of actuators are all held in center valve positions thereof, can perform automatic idling control such that the speed of an engine is maintained at an idling speed of a predetermined low rpm.
b) Description of the Related Art
As prior art of this type, there is one disclosed in Japanese Utility Model Publication (Kokoku) No. HEI 3-52284. FIG. 3 is a hydraulic circuit diagram illustrating this conventional engine speed control system as arranged in a construction machine.
The conventional engine speed control system is arranged in association with a hydraulic circuit of the construction machine. The hydraulic circuit is provided with an engine 40, plural main hydraulic pumps driven by the engine 40, specifically a first main pump 41 and a second main pump 42, plural actuators driven by pressure fluid supplied from these first main pump 41 and second main pump 42, respectively, specifically hydraulic system actuators 55-58,62,63, directional control valves connected with the first main pump 41 to control flows of pressure fluid to be supplied to their corresponding hydraulic system actuators 55-58 from the first main pump 41, specifically change-over control valves 51-54, directional control valves connected with the second main pump 42 to control flows of pressure fluid to be supplied to the corresponding hydraulic system actuators 62,63,58 from the second main pump 42, specifically change-over control valves 59-61, and a reservoir 77.
Of the above-mentioned hydraulic system actuators 55-58,62,63, the hydraulic system actuator 55 constitutes one of a pair of travel motors, the hydraulic system actuator 62 constitutes the other travel motor, and the hydraulic system actuators 56,57,58,63 constitute actuators other than those mentioned above.
As has been mentioned above, the hydraulic actuator 55 which constitutes the one travel motor is connected with the first main pump 41, while the hydraulic actuator 62 which constitutes the other travel motor is connected with the second main pump 42.
The conventional engine speed control system arranged in association with the hydraulic circuit as described above is provided with an engine control mechanism, which performs automatic idling control to set the speed of the engine 40 at an idling speed of a predetermined low rpm and can also perform control to cancel the above-mentioned automatic idling control. This engine control mechanism comprises, for example, a speed governor 67 which in turn, is equipped inter alia with a governor lever 69 for controlling a quantity of fuel to be injected and a spring 68 by which the governor lever 69 is biased.
The conventional engine speed control system is also provided with a pilot pump 64 capable of supplying a pilot pressure, a pilot line 65 for guiding the pilot pressure supplied from the pilot pump 64, pilot valves 70-76 arranged independently in the pilot line 65 in association with the above-mentioned change-over control valves 51-54,59-61, respectively, such that the pilot valves are operated in association with their corresponding change-over control valves, and a pressure detection device for detecting a pressure developed in the pilot line 65 upon change-over of at least one of these pilot valves 70-76 and then outputting it as a detection signal to the above-mentioned speed governor 67, for example, a line 78.
When the change-over control valves 51-54,59-61 are all held in their center positions, for example, as shown in FIG. 3, the pilot line 65 is in communication with the reservoir 77 so that the pilot line 65 has the reservoir pressure. This reservoir pressure is supplied to the speed governor 67 via the line 78. By the force of the spring 68 which is greater than the force of the reservoir pressure, the governor lever 69 assumes a position "a" in FIG. 3. As a consequence, the speed of the ending 40 is maintained at an idling speed of a predetermined low rpm, thereby realizing a reduction in fuel consumption.
Let's assume that, to drive one of the hydraulic system actuators 55-58,62,63, for example, the corresponding one of the change-over control valves 51-54,59-61 is operated. One of the pilot valves 70-76, said one pilot valve corresponding to the operated change-over control valve, is changed over concurrently and as a consequence, a pressure is developed in the pilot line 65 at a part between the change-over control valve 74, which is arranged on a most upstream side in the pilot line 65, and the pilot pump 64. This pressure is supplied to the governor 67 via the line 78, so that the spring 68 is compressed and the governor lever 69 assumes a position "b" in FIG. 3. Accordingly, control is performed to cancel the control that the engine speed be maintained at the idling speed as mentioned above, namely, the automatic idling control. As a result, the engine 40 can be driven at a desired rpm, for example, a rated rpm or the like.
In the above-described conventional art, the change-over control valve 51 for controlling drive of the hydraulic actuator 55, which constitutes the one travel motor, and the change-over control valves 52-54 for controlling drive of the hydraulic system actuators 56-58, which constitute the plural other actuators, are connected to the first main pump 41, whereas the change-over control valve 59 for controlling drive of the hydraulic system actuator 62, which constitutes the other travel motor, and the change-over control valves 60,61 for controlling drive of the hydraulic system actuators 63,58, which constitute the other actuators, are connected to the second main pump 42.
Accordingly, upon single operation of travelling, the pressure fluid delivered from the first main pump 41 is supplied to the change-over control valve 51 for the hydraulic system actuator 55 and the pressure fluid delivered from the second main pump 42 is supplied to the change-over control valve 59 for the hydraulic system actuator 62, so that the single operation of travelling can be performed as desired.
However, when it is desired to perform, for example, a combined operation of travelling and another operation, which is to be performed by driving the hydraulic system actuator 56, from the state of such a single operation of travelling, the pressure fluid delivered from the first main pump 41 is supplied to both of the change-over control valve 51 for the hydraulic system actuator 55, which constitutes the one travel motor, and the change-over control valve 52 for the hydraulic system actuator 56, while the pressure fluid delivered from the second main pump 42 is supplied only to the change-over control valve 59 for the hydraulic system actuator 62 which constitutes the other travel motor. During such a combined operation of travelling and another operation by another actuator, it is therefore impossible to assure independence for the travelling, leading to a potential problem that the construction machine may be caused to travel in a zigzag.